Far-infrared Spectra Research Articles

Hydrogen bond-induced responses in mid- and far-infrared spectra of interfacial water at phospholipid bilayers

The vibrational spectra of interfacial water at phospholipid bilayers is critical to the understanding of cell membrane functions. In this work, we employ molecular dynamics simulations to explore simultaneous responses in both mid- and far-Infrared (IR) spectra of water at the interface with 3-palmitoyl-2-oleoyl-d-glycero-1-phosphatidyl choline (POPC) bilayers. Our results show that the interfacial water can exhibit both a red shift in the mid-IR region for the O–H stretching mode and a blue shift in the far-IR region due to the vibrational mode of water-water hydrogen bonds (HBs). More interestingly, our simulation results reveal that such shifts in both mid- and far-IR regions are essentially induced by the interfacial water molecules that form HBs with the POPC. In particular, the HBs between the interfacial water and the POPC are strong, especially for those with the non-bridging oxygen atoms connecting the phosphorus atom. Such preferential leads to the considerably enhanced HB strength which in return promotes shifts in the mid- and far-IR regions.

Multispectral Video Fusion for Non-Contact Monitoring of Respiratory Rate and Apnea.

Continuous monitoring of respiratory activity is desirable in many clinical applications to detect respiratory events. Non-contact monitoring of respiration can be achieved with near- and far-infrared spectrum cameras. However, current technologies are not sufficiently robust to be used in clinical applications. For example, they fail to estimate an accurate respiratory rate (RR) during apnea. We present a novel algorithm based on multispectral data fusion that aims at estimating RR also during apnea. The algorithm independently addresses the RR estimation and apnea detection tasks. Respiratory information is extracted from multiple sources and fed into an RR estimator and an apnea detector whose results are fused into a final respiratory activity estimation. We evaluated the system retrospectively using data from 30 healthy adults who performed diverse controlled breathing tasks while lying supine in a dark room and reproduced central and obstructive apneic events. Combining multiple respiratory information from multispectral cameras improved the root mean square error (RMSE) accuracy of the RR estimation from up to 4.64 monospectral data down to 1.60 breaths/min. The median F1 scores for classifying obstructive (0.75 to 0.86) and central apnea (0.75 to 0.93) also improved. Furthermore, the independent consideration of apnea detection led to a more robust system (RMSE of 4.44 vs. 7.96 breaths/min). Our findings may represent a step towards the use of cameras for vital sign monitoring in medical applications.

Strong anion–anion hydrogen bond in the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate

This work discusses the remarkable differences in the mid- and far-infrared regions of the spectrum of the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate, [C2C1im][HSO4], in comparison to ionic liquids based on other anions. The infrared spectrum unveils the occurrence of anion–anion hydrogen bond in [C2C1im][HSO4]. The band related to the anion OH stretching mode, ν(OH), covers a large frequency range (2000–3400 cm−1) and exhibits the spectral pattern of strongly hydrogen bonded systems. The far-infrared (FIR) spectrum of [C2C1im][HSO4] also exhibits the signature of the anion–anion interaction, that is, the stretching mode of the hydrogen bond, ν(O…O), at ~170 cm−1. Hydrogen bond between [HSO4]− anions also occurs in ionic liquids containing imidazolium cation with longer alkyl chain ([C4C1im]+) or tetraalkylammonium cation. The vibrational frequencies of ν(OH) and ν(O…O) modes satisfy empirical correlations that characterize the anion–anion interaction in [C2C1Im][HSO4] as a strong hydrogen bond.

High-temperature structural phase transition and infrared dielectric features of La2CoMnO6

In this paper, we present the temperature-dependent far-infrared reflectivity spectra of partially ordered magnetodielectric La2CoMnO6 from room temperature to 673 K. We demonstrate a clear first-order structural phase transition (SPT) from a monoclinic structure with P21/n symmetry to a rhombohedral phase with R3¯ symmetry from the behavior of the polar phonon modes at TC ∼ 590 K. The temperature-dependent transversal and longitudinal phonon branches, dielectric strengths, and damping of the strongest dielectric modes support the significant contribution of the phonon modes to the SPT and reveal an important lattice anharmonicity, especially for the low-frequency modes. Also, these investigations showed that structural ordering does not inhibit the SPT and provided valuable information regarding the polar phonons and their effects on the intrinsic dielectric constant in double perovskites and related compounds.

Design of efficient graphene plasmonic coupling circuits for THz applications

PurposeThe coupling characteristics between adjacent circuits are crucial for their efficient design in terms of electromagnetic compatibility features. Specifically, either the wireless power transfer can be enhanced or the interference can be limited. This paper aims to the extraction of the coupling characteristics of surface plasmon polariton waves propagating onto graphene layers to facilitate the telecommunication system design for advanced THz applications.Design/methodology/approachThe surface conductivity of graphene is described at the far-infrared spectrum and modelled accurately by means of a properly modified finite-difference time-domain) scheme. Then, a series of numerical simulations for different coupling setups is conducted to extract an accurate generalised parametric coupling model that is dependent explicitly on the fundamental propagation features of graphene.FindingsThe coupling coefficients of two basic waveguiding setups are examined thoroughly. The initial one includes two parallel graphene layers of infinite dimensions, and it is observed that the coupling is influenced via the ratio between their distances to the confinement of the surface wave. The second scenario is composed of graphene microstrips that are parallel to their small edge, namely, microstrip width. The extracted numerical results indicate that the coupling coefficient depends on the ratio between widths to wavelength.Originality/valueThe accurate extraction of the generalised coupling coefficients for graphene surface wave circuits is conducted in this work via an adjustable numerical technique for a novel family of plasmonic couplers. It is derived that only the fundamental propagation features of graphene, such as the wavelength and the confinement of the surface waves, have an effect on the coupling calculation, thus enabling a consistent electromagnetic compatibility study.

Structural and optical properties of ZnO–Al2O3 nanopowders prepared by chemical methods

The nanopowders of (ZnO)1–x(Al2O3)x, where x ranges from 0 to 0.7, were obtained by two chemical methods: the co-precipitation/calcination and hydrothermal synthesis. The first assessment of structural and optical properties of the obtained nanopowders was undertaken by the SEM, XRD, Raman and far-infrared spectroscopy, which was followed by the photoluminescence spectroscopy at room temperature. The obtained far-infrared reflectivity spectra were analyzed using the fitting procedure. The dielectric function of ZnO–Al2O3 nanopowders was modeled by the Maxwell-Garnet formula under the assumption that the nanopowders are a mixture of homogenous spherical inclusions in air. The combined plasmon-LO phonon modes (CPPM) were observed in the far-infrared reflection spectra. The photoluminescence spectra contain emissions related to the presence of ZnO, ZnAl2O4, and AlOOH phases in the nanomaterial, which is in agreement with the results obtained by other experiments.

Pseudo-Optical Modes in Room-Temperature Ionic Liquids.

The terahertz spectrum encodes information about dynamics, structure, and intermolecular interactions of liquids being probed both experimentally and computationally by techniques such as Raman and far-infrared spectroscopies and molecular dynamics (MD) simulation. In the case of room temperature ionic liquids (RTILs), there has been a debate whether a mode observed at about 1.5-2.7 THz (50-90 cm-1) is due to a quasi-lattice structure or the formation of complex ions. Here we show through the analysis of Raman and far-infrared spectra and MD simulation of a typical RTIL that this mode has a collective optic-like character. Then, employing a simple model based on the theory for optical phonons in crystals, we show that a correlation between the frequency of this mode and material parameters holds for different RTILs. These results, which encompass a wide range of samples, reinforce a quasi-lattice view of the liquid phase.

Synchrotron-Based High Resolution Far-Infrared Spectroscopy of trans-Butadiene.

The high resolution far-infrared spectrum of trans-butadiene has been reinvestigated by Fourier-transform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm-1, ν10 and ν24, have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν24 analysis was rather straightforward, ν10 exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν9 + ν12 state. Effective rotational constants have been derived for both the v10 = 1 and v24 = 1 states. Since only one weak, infrared active fundamental band (ν23) of trans-butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive gauche conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the trans form.

Polarization insensitive graphene-based tunable frequency selective surface for far-infrared frequency spectrum

Abstract We presented the numerical analysis of the polarization insensitive tunable T-shaped graphene-based frequency selective surface for the far infrared frequency spectrum. The proposed frequency selective surface is tunable for the different values of the chemical potential which can be controlled by the external biasing. The proposed structure tunable over the frequency range varying between 1 THz and 3 THz. The proposed structure is also numerically investigated for the complementary behaviour of the proposed design. The basic parameters such as transmittance, reflectance, reflected wave polarization are investigated for the proposed designs. The wide-angle incidence response is also investigated for the proposed structure. It is also achieved that the graphene frequency selective surface is polarization insensitive for the TE and TM excited mode. The dipole moment of the different resonance points are also investigated to check the reflection amplitude and the charge current variation during the polarization effect. The proposed results of frequency selective surface structure can be applied for designing the basic photonics components like polarizer, sensor, THz filter, metamaterial which will become an important component of THz applications.

Patterns in synchrotron near-free-rotor FIR spectra of CH3OH and CD3OH – The tau of methanol

Analysis of high-resolution synchrotron far-infrared spectra of CH3OH and CD3OH has been extended up to high vt = 3 to 5 torsional levels in the near-free rotor region. In this region, the free-rotor angular momentum quantum number m becomes the natural label for the torsional levels, and sub-bands with Δm = 0 have torsional overlap matrix elements close to unity giving rise to surprisingly strong features in the spectrum. A simple graphical picture of free rotor parabolas gives a convenient spectral predictor for the locations of the strong sub-bands, and shows that they display systematic patterns when classified by Dennison’s torsional index τ instead of the customary A and E symmetries. The initial lines of observed near-free-rotor sub-bands with the same τ connections fall in tight τ-groups encompassing a wide range in K with a constant separation between groups of different Δτ, confirming a pattern predicted from the earliest literature on internal rotation in methanol. Together with previously known data, results for newly assigned sub-bands illustrate the τ-grouping and extend the information database of ground-state methanol term values.

Extending KIDs to the Mid-IR for Future Space and Suborbital Observatories

The galaxy evolution probe (GEP) is a concept for a probe-class space observatory to study the physical processes related to star formation over cosmic time. To do so, the mid- and far-infrared (IR) spectra of galaxies must be studied. These mid- and far-IR observations require large multi-frequency arrays, sensitive detectors. Our goal is to develop low NEP aluminum kinetic inductance detectors (KIDs) for wavelengths of 10–400 {upmu }{{hbox {m}}} for the GEP and a pathfinder long-duration balloon (GEP-B) that will perform precursor GEP science. KIDs for the lower wavelength range (10–100 {upmu }{{hbox {m}}}) have not been previously implemented. We present an absorber design for KIDs sensitive to wavelengths of 10 {upmu }{{hbox {m}}} shown to have around 75–80% absorption efficiency through ANSYS HFSS (high-frequency structure simulator) simulations, challenges that come with optimizing our design to increase the wavelength range, initial tests on our design of fabricated 10 {upmu }{{hbox {m}}} KIDs, and theoretical NEP calculations.

Far-Infrared Synchrotron Spectroscopy of a Potentially Important Interstellar Isotopologue of Vinyl Alcohol: CH2CHOD.

The far-infrared spectrum (100-500 cm-1) of a d1 isotopologue of the astrophysically important molecule, vinyl alcohol, is reported. We observed several low energy (OD) torsional bands: the fundamental and first two hot bands of the syn rotamer and the fundamental and first hot band of the (higher energy) anti rotamer. While the bands corresponding to the anti rotamer are somewhat obscured by rotational lines of water (making a full spectroscopic analysis unfeasible at this stage), the syn-vinyl alcohol bands are not, and a global fit was performed that included 4404 distinct infrared lines assigned in this work, in addition to 59 previously reported microwave lines. This simultaneous analysis of the torsional fundamental, torsional hot band, and pure rotational band of syn-vinyl alcohol allowed for determination of spectroscopic parameters in the first two torsionally excited states and for refinement of them in the ground state. These parameters should be useful in searches for both cold and warm CH2CHOD in interstellar molecular clouds.

Quantum chemical verification of identification of three phenol-type antioxidants by far-infrared absorption spectroscopy

Three different kinds of phenol-type antioxidants were added in low-density polyethylene sheets. Far-infrared and mid-infrared absorption spectra were measured for these sheets and the sheet with no antioxidants to obtain spectra of antioxidants. Furthermore, chemical structures of the antioxidants were optimized by quantum chemical calculations and possible vibrational modes and their energies were calculated numerically. As a result, it was found that our calculations simulate absorption spectra of the antioxidants successfully and that their causal vibrations in the mid-infrared range are consistent with those reported in the literature. Furthermore, it has become clear that all the major far-infrared absorption peaks are due to skeletal vibrations. This clearly explains our previous results that the three antioxidants with very similar structures can be identified by the far-infrared absorption spectroscopy.

The infrared spectra of protic ionic liquids: performances of different computational models to predict hydrogen bonds and conformer evolution.

The temperature dependence of the far- and mid-infrared spectrum of two prototypical protic ionic liquids (PILs) sharing a common trialkylammonium cation, but having different anions, is investigated. The exploitation of both the FIR and MIR ranges provides complementary information about the microscopic configurations and the intermolecular interactions, which determine the structure and the properties of ILs. The analysis of the data collected for all the measured frequencies in a wide temperature range reveals several phase transitions and allows the evaluation of the conformer distribution in the different physical states. The difference in the average energy between the H-bonded configurations and the dispersion-governed ones was also determined for the two PILs. Moreover, a computational model for ionic couples based on the ωB97X-D functional and a polar solvent is here successfully exploited for the description of the hydrogen bonding between anion and cation. For the attribution of vibrational lines of the conformers of the cation, the picture based on single ion calculations at the B3LYP level is more valuable and provides better agreement with the experiments.

Far-Infrared Investigation of the Benzene-Water Complex: The Identification of Large-Amplitude Motion and Tunneling Pathways.

The far-infrared spectrum of the weakly OH···π hydrogen-bonded benzene-water complex has been studied in neon and argon matrices, below 30 K. The in-plane water libration has been observed in both neon and argon for H2O and D2O complexed with C6H6 and C6D6 but not for the corresponding complexes involving HDO. Both H2O and D2O can tunnel between the two possible hydrogen bonds. This is not possible for HDO. The reported far-infrared observations have implications for the interpretation of the previously obtained molecular beam microwave spectrum of the benzene-water complex.

Origin of Two Distinct Peaks of Ice in the THz Region and Its Application for Natural Gas Hydrate Dissociation

For liquid water in the far-infrared spectrum, phonons of molecular vibrations constitute two bands with a narrow gap at around 30 meV. Interestingly, there are two distinct peaks for ice in this g...

The prediction of far-infrared spectra for molecular crystals of forensic interest – Phenylethylamine, ephedrine & pseudoephedrine

Synchrotron far-infrared (far-IR) spectra were measured for a representative set of pharmaceutical-type molecular crystals: phenylethylamine hydrochloride salt, ephedrine base and its hydrochloride salt, and pseudoephedrine base and its hydrochloride salt. Resolved spectral profiles in the 30–600 cm−1 range allowed for comparison of absorption bands to simulated spectra obtained by periodic density function theory (DFT) code. The B3LYP-D3/6-311G(d) level of theory returned highly accurate predictions of the experimental far-IR spectra for well-ordered crystalline samples, including an agreement between relative peak intensities almost unprecedented for organic crystal systems of this size. Simulated far-IR spectra for the phenylethylamine hydrochloride and ephedrine base samples do not match to the same degree; potentially due to higher disorder within the solid-state. These comparisons to theory generally allowed for confident assignment of prominent absorption bands to low-lying vibrational modes. The experimental spectra (including those for related illicit drug compounds: amphetamine sulphate, methamphetamine hydrochloride, MDA hydrochloride, and MDMA hydrochloride), peak assignments, and relative band intensities are compiled and evaluated for their inclusion in spectral libraries and their application to chemometric analysis for mixed compounds of pharmaceutical and forensic interest.

The New EXor Outburst of ESO-Hα 99 Observed by Gaia ATLAS and TESS

Abstract We report photometry and spectroscopy of the outburst of the young stellar object ESO-Hα99. The outburst was first noticed in Gaia alert Gaia18dvc and later by the Asteroid Terrestrial-impact Last Alert System (ATLAS). We have established the outburst light curve with archival ATLAS orange filter photometry, Gaia data, new V-band photometry, and J, H, and K s photometry from the Infrared Imaging System (IRIS) and the United Kingdom Infrared Telescope (UKIRT). The brightness has fluctuated several times near the light curve maximum. The Transiting Exoplanet Survey Satellite (TESS) satellite observed ESO-Hα 99 with high cadence during one of these minor minima and found brightness fluctuations on timescales of days and hours. Imaging with UKIRT shows the outline of an outflow cavity, and we find one knot of emission, now named MHO 1520, on the symmetry axis of this nebula, indicating recent collimated outflow activity from ESO-Hα 99. Its pre-outburst SED shows a flat far-infrared spectrum, confirming its early evolutionary state and its similarity to other deeply embedded objects in the broader EXor class. The pre-outburst luminosity is 34 L ⊙, a much higher luminosity than typical EXors, indicating that ESO-Hα 99 may be a star of intermediate mass. Infrared and optical spectroscopy show a rich emission-line spectrum, including H i lines, strong red Ca ii emission, as well as infrared CO bandhead emission, all characteristic EXors in the broadest sense. Comparison of the present spectra with an optical spectrum obtained in 1993, presumably in the quiescent state of the object, shows that during the present outburst the continuum component of the spectrum has increased notably more than the emission lines. The Hα equivalent width during the outburst is down to one-half of its 1993 level, and shock-excited emission lines are much less prominent.

Novel temperature stable NiSnTa2O8 microwave dielectric ceramics with trirutile structure

A novel low-loss and temperature stable NiSnTa2O8 ceramic with trirutile structure was prepared using traditional solid-state method. The structure-performance relationships were investigated by Rietveld refinement, chemical bond theory and far-infrared spectrum. The results show that the relative densities play a dominant role in the change of dielectric constant. Theoretical dielectric constant calculated via bond theory, Clausius-Mossotti equation and fitted result of far-infrared spectrum are close to experimental value. Ta–O bonds with greatest bond ionicity and bond energy have the primary contributions to dielectric polarizabilities and dielectric loss. The optimal microwave dielectric performances of NiSnTa2O8 ceramics were obtained: εr ∼21.04, Q×f ∼31328 GHz and τf = −2.63 ppm/°C at 1425 °C.

The O2 far-infrared absorption spectrum between 50 and 170 cm-1

The oxygen absorption spectrum in the 50–170 cm-1 spectral range is studied at the AILES beam line of the SOLEIL synchrotron with a Fourier transform spectrometer equipped with a 151-m multipass gas cell. The spectrum recorded at room temperature (23.15 °C) with a pressure of 19.76 Torr is formed by weak pure rotational magnetic dipole transitions. Line parameters of 26 lines are derived and compared to literature values. In particular, measured line intensities confirm the 45 years-old previous values reported by Boreiko et al. (J. Quant. Spectrosc. Radiat. Transfer 32 (1984)109–117). The agreement with HITRAN intensities (within 1% level for the strongest lines) indicates that the 20% HITRAN error estimate was overly cautious: we show that the error is more likely within 2%.